Radio Frequency Identification (“RFID”) technology comprises two elements: a transponder (hereafter “tag”), which is generally a small, paper thin computer chip with an antenna which stores data, and a transceiver which utilizes a radio signal (hereafter “RF”) in the approximate 800-930 Mhz UHF range to read the data from the tag. Active RFID tags contain a power source, such as a battery, and can actively transmit the tag's stored data. Passive RFID tags cannot transmit by themselves, and require a RFID transceiver to provide power via radio signals transmitted by the RFID transceiver. As passive RFID tags pass by an RFID transceiver, the transceiver powers the tag and reads the data stored on the tag.
Large retail companies such as WAL-MART® find passive RFID tags advantageous over optical barcodes for inventory tracking RFID tags have two distinct advantages over traditional optical barcodes: RFID tags can store more information, and RFID tags do not require line-of-sight readings.
A problem in the implementation of passive RFID technology for inventory tracking arises from dead spots. Dead spots can be caused by dense packing of passive RFID tags where inventory materials shield the tags and interrupt signal transmission. For example, in a pallet containing sixty cases of canned goods, where each case has a passive RFID tag, the metal cans around the periphery of the pallet shield the RFID transceiver's signal. Even if the RFID transceiver successfully reads fifty of the sixty RFID tags located on the pallet, the RFID technology failed as a tracking and inventory method because the RFID technology gathered incomplete and inaccurate information.
Dead spots can be avoided by placing passive RFID tags to avoid shielding problems. In addition, RFID repeaters, placed within the packed pallet, can ensure that the RFID transceiver's signal reads all of the passive RFID tags on the pallet. But in order to be sure that all of the passive RFID tags on the pallet are read, the placement of the passive RFID tags and RFID repeaters, if any, must be tested.
One known method for testing the placement of RFID tags uses heat-sensitive liquid crystals (“LCDs”) that change color when exposed to certain radio frequencies. LCDs that are sensitive to the RF frequency transmitted by an RFID transceiver can act as markers, showing where the reach of the RFID signal. Furthermore, light emitting diodes (“LEDs”) affixed to products or product packaging may be coupled with a radio frequency receiver to verify the scanning of passive RFID tags. Using these known methods, RFID tag locations may be tested by trial and error.
A need exists for a more efficient (non-trial and error) method to determine placement of passive RFID tags or RFID repeaters in densely packed product and product packaging storage unit to ensure RFID transceivers can read all the passive RFID tags in the storage unit.